The present invention relates to spread spectrum communication systems; and more specifically to code-division multiple-access communication systems having multiple carriers serving a common geographic area.
Spread spectrum communication systems, such as code division multiple access systems, may use multiple carriers to support traffic requirements of a common geographic area. As used herein, common geographic area refers to a cell site, a sector of a cell site, a cluster of cells, or a cluster of sectors, which is served by more than one radio frequency carrier. Each radio frequency carrier supports multiple traffic channels. Because each subscriber within the common geographic area may be supported by multiple carriers, the communications system must decide how to allocate each subscriber between at least two radio frequency carriers.
One approach used to select a radio frequency carrier for a subscriber entails selection of a carrier with the greatest physical equipment support for traffic channels. For example, a first carrier may have a first level of equipped traffic channels at a cell site serving a subscriber. Meanwhile, a second carrier may have a second level of equipped traffic channels which is greater than the first level. Because the second level of equipped traffic channels exceeds the first level, the communications system transfers the subscriber to the second carrier. However, maximizing the use of the equipment per carrier may lead to unnecessary hand-offs. In the above example, if the first carrier was not fully loaded, no hand-off would be truly mandatory.
Hand-offs between carriers during call-setup may contribute to call setup failures. For instance, call-setup failure may occur because of differential interference of the second carrier with respect to first carrier. The differential interference may contribute to poor reception of downlink or uplink radio frequency signals required to setup or maintain calls. The differential interference may result because adjacent cell sites or surrounding cell sites dynamically add to the background noise and interference about a carrier frequency during movement of mobile subscribers. Even if subscribers are not mobile, as in a wireless local loop (WLL) configuration, traffic variations over time and carrier assignments may influence the interference present at a cell of interest. Accordingly, the first carrier often has a different measurable interference than the second carrier at the cell of interest within a cellular communications system.
Assume for illustrative purposes that the first carrier is a transferring carrier and the second carrier is a transferee carrier. If the second carrier has a higher interference level than the first carrier, the subscriber""s transmit power level may be too weak to adequately compensate for the higher interference, impeding reliable transmission on the uplink over the second carrier. Thus, the base station for receiving the second carrier may be unable to receive the traffic channel transmission from the subscriber because of differential interference. The base station may time out waiting for a response or transmission from the subscriber. While power control algorithms for the subscribers can increase power levels to compensate for increased background noise and interference, indiscriminately increasing power during call setup may reduce the overall capacity of the communications system by reducing the signal-to-noise ratio for other subscribers sharing the communications system.
Most code-division multiple-access (CDMA) systems use a soft-hand off for intra-carrier hand-offs, but use a hard-hand off for inter-carrier hand-offs. Intra-carrier hand-offs include hand-offs between adjacent cells, adjacent sectors, other cells, or other sectors that maintain communications over a single carrier on the same frequency spectrum as the subscriber moves throughout the communication system. Inter-carrier hand-offs are hand-offs or transfers between carriers on different frequency spectrums. An inter-carrier hand-off may occur within one cell or one sector, even if the subscriber remains stationary.
Soft hand-offs are considered more reliable than hard hand-offs, because soft-hand-offs use channel resources on a group of cell sites to improve radio frequency coverage reliability. Hard hand-offs provide a somewhat abrupt transition, because the transferring carrier and the transferee carrier generally do not enhance each others radio frequency coverage during a hand-off.
Thus, a need exists for reducing dropped calls and call-setup failures associated with inter-carrier hand-offs and hard hand-offs.
Even if inter-carrier transfers do not contribute to call setup failures, inter-carrier transfers place burdens on the resources of the communication system. Inter-carrier transfers use processing resources of the base station controller. Thus, a need exists for reducing the processing burden of inter-carrier handoffs on the base station controller. Further, a need exists for improving the reliability of communications systems having multiple carriers and using inter-carrier handoffs.
In accordance with a method of the present invention, a communications system selects a radio frequency carrier for a subscriber based upon a radio frequency loading factor associated with each carrier and a predetermined threshold for evaluating loading factors. The predetermined threshold may be set by an operator of the communications system or automatically by the communications system.
While the predetermined threshold is preferably based on actual measurements of interference on different carriers, the predetermined threshold does not need to be limited to considerations of interference or noise. Accordingly, the predetermined threshold adds flexibility in allocating carriers to subscribers by allowing an operator to define the predetermined threshold based upon any relevant communication system parameters. For example, the predetermined threshold may be defined to represent differential interference measurements, the presence of border cells, the extent of physical equipment per carrier, or any combination of the foregoing communication system parameters. The predetermined threshold advantageously may be further optimized based on traffic conditions or other factors specific or idiosyncratic to individual communications systems.
In a preferred example of the method of the invention, the predetermined threshold is established to prevent inter-carrier handoffs where the interference differential between carriers is likely to cause a call setup failure during an inter-carrier hand-off. Thus, transfers of subscribers are minimized to reduce dropped call rates and other call setup failures. Moreover, the method tends to reduce the use of processing resources of the base station subsystem, including a base station controller, by reducing unnecessary handoffs consistent with an appropriate selection of the predetermined threshold.